An alkali-free glass is employed as a transparent glass substrate for a liquid crystal display or the like. Such alkali-free glass is required to be free from bubbles constituting defects in the display.
The alkali-free gla is melted at a higher temperature in comparison with glass containing an alkali melted component, because of a higher viscosity of the molten glass. In melting the alkali-free glass, a vitrifying reaction usually takes place at from 1200 to 1400° C., and bubble reduction and homogenization are executed at a high temperature of 1500° C. or higher. Therefore, it is necessary to utilize a fining agent capable of releasing fining gas at least in a high temperature region where the bubble reduction and the homogenization are executed. Because of such situation, As2O3 capable of generating fining gas in a wide temperature range has been widely employed as the fining agent for the alkali-free glass. However, As2O3 is a hazardous material and it is desired to reduce or totally replace such material.
Therefore, methods utilizing SO3, Sb2O3, SnO2, Cl2 or the like as an alternative fining agent for As2O3 are proposed (for example, patent references 1 and 2).    [Patent Reference 1]
JP-A-11-43350    [Patent Reference 2]
JP-A-2000-159541